I go for a bit of a formula with mine.
If it’s a regular then I just have their name and a client number that links back to an excel file where I keep track of all my clients details.
If its someone I don’t want to see again its just NO followed by client number.
If its anyone else then its name, client number , rating number (1-5 depending on how enjoyable the whole experience was) plus a few letters to represent what was most memorable about them:
GB or GH Good Body and/or Good Hygiene
bb or bh bad body and/or bad hygiene
ns or lmc no show or last minute cancelled
bt bad timing (either turned up late, tried to overstay, etc)
b boring
t talker